Industrial robots have become an indispensable part of modern manufacturing. Whether transferring semiconductor wafers from one process chamber to another in a cleanroom or cutting and welding steel on the floor of an automobile manufacturing plant, robots perform many manufacturing tasks tirelessly, in hostile environments, and with high precision and repeatability.
In many robotic manufacturing applications, the considerable cost of an industrial robot is amortized over a variety of tasks by providing different tools, or end effectors, that may be coupled to a general-purpose robotic arm. For example, in an automotive manufacturing application, a robot may be utilized to cut, grind, or otherwise shape metal parts during one production run, and perform a variety of spot welding tasks in another. Different welding tool geometries may be advantageously mated to a particular robot to perform welding tasks at different locations or in different orientations. In these applications, a tool changer is used to mate different tools to the robot.
One half of the tool changer, called the master unit, is permanently affixed to a robot arm. The other half, called the tool unit, is affixed to each tool that the robot may utilize. Utilities such as electrical current, air pressure, hydraulic fluid, cooling water, and the like, are fed through cables and plumbing down the robot arm, that terminate at the master unit. Similar cables and plumbing carry the utilities from the tool unit to the particular tool. When the tool changer halves are mated, the utilities are transferred across the changer and made available at the tool. A tool changer thus provides a standard mechanical interface for physically coupling a variety of tools to a robotic arm, as well as providing for the transfer of utilities. Utility and safety concerns dictate that the physical coupling between master and tool units of a robotic tool changer be robust and secure, even in the face of a power outage or loss of a utility such as pneumatic pressure.
While industrial robots tend to be large, highly automated devices, robotic tool changers find utility in other applications. For example, in robot-assisted surgery, a relatively small robot arm positions surgical tools in a pre-defined “safe zone.” Surgeons then operate using the tools, while the robot prevents the tools from moving outside of the pre-defined safe zone of operation. In such applications, due to size, weight, cost, and among other constraints to properly clean and sterilize the tool, it may be advantageous for tools to be attached, and for a robotic tool changer to be actuated (that is, or moved between coupled and decoupled states) manually.
The use of ball members, urged by a piston against an inclined surface, to lock the master and tool units together is known in the art. For example, U.S. Pat. No. 4,696,524 (incorporated herein by reference) discloses a plurality of ball members contained within the master unit, and circumferentially arranged around a central axis. Extending from the master unit, along this axis, is a piston member having an inclined surface operative to contact the ball members and urge them outwardly as the piston advances axially. The ball members contact a surface in the tool unit disposed at an angle such that outward force induced on the ball members by the piston generates an “upward” force component that presses the angled surface, and thus the entire tool unit, against the master unit.
U.S. Pat. No. 5,211,501 (incorporated herein by reference) discloses a similar piston and ball member arrangement, with an improved piston/ball member contact surface. This patent discloses a multifaceted contact surface comprising an initial tapered contact surface for first contacting the ball members and moving them outward and into contact with an angled surface of the tool unit. A flat—i.e., parallel with the piston axis—failsafe surface is adjacent the initial tapered surface. A tapered locking surface, at an angle with respect to the axis of less than that of the initial actuating surface, is adjacent the failsafe surface.
For the following discussion, assume the master unit is oriented over the tool unit, with the interface plane between the two modules parallel with the horizon. As the piston member advances axially (downwardly) into the tool unit, the initial contact surface contacts the ball members and moves them radially outward (horizontally) into the tool unit. At the extent of the piston's axial movement, the final tapered surface presses each ball member outwardly against an angled surface in the tool unit. This angled surface tapers inwardly, toward the piston axis, as it approaches the master unit. Each ball member, urged outwardly by the tapered locking surface of the piston member, presses against the tool unit angled surface with a resultant force that can be decomposed into horizontal (outward) and vertical (upward) components. The vertical component of force presses the tool unit upward and locks the tool unit to the master unit.
The ball members press inwardly against the piston with equal and opposite force. Since the tapered locking surface is angled with respect to the piston axis, the force exerted by each ball member is a resultant force that can also be decomposed into horizontal (inward) and vertical (upward) components. In the event of a loss of force actuating the piston, the vertical component of force exerted by the ball members urges the piston upwardly. As the piston moves upwardly, the balls are free to move inwardly, pressing with less force on the tool unit angled surface and tending to decouple the master and tool units. For safety, a failsafe surface is interposed between the piston initial contact surface and the tapered locking surface, both of which are tapered. The failsafe surface is vertical—i.e., parallel with the piston axis. During a power loss, force exerted by the ball members may move the piston slightly upwards, until the ball members contact the failsafe surface. Since the failsafe surface is vertical, the resultant force exerted by the ball members is normal, i.e., horizontal, and includes no vertical component. This prevents force from the balls on the piston from further retracting the piston into the master unit and further decoupling the modules, without some positive actuation of the piston in that direction. Accordingly, the tool unit remains coupled to the master unit when piston actuating power is lost.
U.S. Pat. No. 7,252,453 (incorporated herein by reference) discloses a similar piston and ball member arrangement, with a piston member contact surface having a tapered locking surface at an angle with respect to the axis greater than that of the initial actuating surface.
Pending patent application Ser. No. 11/374,706 (incorporated herein by reference) discloses a similar piston and ball member arrangement, with a piston member contact surface having a failsafe surface that includes a lip, or protrusion, which actively opposes retracting motion of the piston. When the master and tool units are coupled together—that is, when the ball members are fully extended by the tapered locking surface and pressing against the tool unit angled surface—the protrusion on the failsafe surface is past (below) the ball members. In the event of loss of piston actuating power, the force exerted by the ball members on the tapered locking surface tends to decouple the master and tool units, as described above. This tendency is neutralized by the failsafe surface being parallel to the piston axis, thus not supporting any component of force in the axial direction. The protrusion provides an additional assurance that the piston cannot retract into the master unit. Moving the protrusion past the ball members requires a positive retracting force on the piston, since the balls must momentarily be pressed yet further against the tool unit angled surface for the protrusion to pass. The protrusion may comprise a raised surface, or the lip of a depression in the failsafe surface into which the ball members nestle.
Pending provisional patent application Ser. No. 60/789,004 (incorporated herein by reference), discloses a variety of similar piston and ball member arrangements, wherein the piston is actuated between retracted and extended positions by electrical power and various power transmission systems and gear trains.
In all of the above examples, the ball members are moved outwardly against the tool unit's angled surface by axial motion of a piston. This requires sufficient room in the master tool unit above the piston to house the piston in the retracted position. If the ball members could be actuated outwardly and forced against the tool unit angled surface without requiring axial motion of a piston, the master tool unit may be designed with a more compact, lower profile shape.